In the telecommunications art, it is often necessary to amplify high-frequency analog signals for transmission across a medium such as copper wire. This is commonly achieved by using electronic circuits known as power amplifiers, which are biased by a supply voltage source. In an ideal scenario, power amplifiers operate at high speeds, produce very low distortion and are efficient in their power consumption.
Unfortunately, conventional power amplifiers are not capable of satisfying these three requirements. The most common problem is that the maximum output voltage falls several volts short of the supply voltage "rails", hence leading to poor power efficiency and effectively curtailing the input voltage range that can be linearly amplified by the power amplifier. When the input voltage exceeds this limit, certain transistors in the power amplifier may enter saturation, causing severe distortive effects.
Moreover, due to its random nature in many applications, the input voltage will fluctuate and drop to a level which requires previously saturated transistors to exit saturation and operate normally. Since recovery from saturation is an unstable process, the useful operating frequency range and dynamic range of the power amplifier are both severely limited.
For example, it is known that with supply voltage rails of plus and minus 10 volts, typical amplifiers (such as the OPA 642 from BurrBrown, the MAX 4187 from Maxim or the CLC 505 from National Semiconductor) are rather inefficient, as they are only capable of producing an output voltage swing of approximately 3 volts peak-to-peak while meeting distortion requirements at high frequencies of operation.
Thus there is a need for a power amplifier configuration that operates with high power efficiency, while continuing to provide high-speed functionality combined with very low distortion.